(1) Field of the Invention
This invention relates to a frequency modulation (hereinafter, FM) multiplex demodulation device, by which a (Lxe2x88x92R) signal multiplexed by a subcarrier at 38 kHz is demodulated to a (L+R) signal contained in a composite signal (FM detected signal) so that a L signal and R signal are obtained.
(2) Description of the Related Art
As to a FM stereo broadcast, a (Lxe2x88x92R) signal is demodulated to a (L+R) signal by a subcarrier at 38 kHz in a FM-detected baseband as shown in FIG. 5A.
So far, a FM multiplex demodulation device shown in FIG. 6 has been used to obtain a L signal and R signal by demodulating such a multiplexed signal.
As shown in FIG. 6, the conventional FM multiplex demodulation device consists of a demodulator 50 for demodulating the multiplexed (Lxe2x88x92R) signal shown in FIG. 5A, a noise detecting section 51 for detecting a noise level contained in the composite signal, an attenuator (ATT) 52 for attenuating the demodulated (Lxe2x88x92R) signal, an adding section 53, and a subtracting section 54.
When the composite signal is received, in a state that a multipass fading exists, noise in a FM-detected output increases, the noise detecting section 51 detects the noise level and changes an attenuation quantity by the attenuator 52 in response to the detected noise level. Therefore, when the noise is not detected, the attenuation quantity by the attenuator 52 becomes to be zero.
As to the demodulator 50, when the subcarrier at 38 kHz is added to the signal shown in FIG. 5A, the (Lxe2x88x92R) signal shown in FIG. 5B is demodulated.
At the adding section 53, the signal shown in FIG. 5B is added to the signal shown in FIG. 5A to obtain 2L signal, while the signal shown in FIG. 5B is subtracted from the signal shown in FIG. 5A to obtain 2R signal, each of which is input into a speaker (not shown in the figure).
The added or subtracted output contains multiplexed subcarrier components, which are not in an audio frequency band and not reproduced from a speaker.
When a noise is detected at the noise detecting section 51, an output at the adding section 53 is equal to
(L+R)+kxe2x80x2(Lxe2x88x92R)=(1+kxe2x80x2)L+(1xe2x88x92kxe2x80x2)R 
and an out put at the subtracting section 54 is equal to
(L+R)xe2x88x92kxe2x80x2(Lxe2x88x92R)=(1xe2x88x92kxe2x80x2)L+(1+kxe2x80x2)R 
where kxe2x80x2 (0xe2x89xa6kxe2x80x2xe2x89xa61) is the attenuation quantity at the attenuator 52. Consequently, it takes effect a reduction in multipass noise, however a feeling of stereo deteriorates at the same time, causing a deterioration of the tone quality.
It is therefore an objective of the present invention to solve the above problem and to provide a FM multiplex demodulation device, in which the feeling of stereo is maintained excellent and the offensive noise to the ears can be reduced.
In order to attain the above objective, the present invention is to provide a FM multiplex demodulation device comprising: a filter for selecting a signal multiplexed by a subcarrier at 38 kHz from a composite signal; a noise detecting section for detecting noises from the composite signal; a ratio changing section for changing a composition ratio between an output signal from the filter and the composite signal, and for outputting a composed signal, the composition ratio being determined in response to the noises detected by the noise detecting section; a demodulating section for demodulating the multiplexed signal with multiplying the output signal from the ratio changing section by the subcarrier; an adding section for adding an output signal from the demodulating section to the composite signal; and a subtracting section for subtracting the output signal from the demodulating section from the composite signal.
The FM multiplex demodulation device further comprises a delaying section for delaying the composite signal for a period of time corresponding to a delay time of the filter, wherein the delayed signal instead of the composite signal is input to the ratio changing section, the adding section and the subtracting section.
The filter filters out a part of a multiplexed signal.
The filter consists of a second-order band-pass infinite impulse response (hereinafter IIR)-type digital filter, a sampling frequency of the digital filter is set to be six times of a center frequency of the pass band, a first-order input feed back coefficient b1 is set as b1=xe2x88x921+2xe2x88x92n, while a second-order input feed back coefficient b2 is set as b2=1xe2x88x922xe2x88x92(nxe2x88x921), where n is 3 or an odd number more than 3.
A zero-order output coefficient a0 is set as a0=2xe2x88x92n and a second-order output coefficient a2 is set as a2=xe2x88x922xe2x88x92n.
A second-order output is subtracted from a zero-order output and the resultant output is multiplied by 2xe2x88x92n.